Continuous casting method and apparatus for controlling freeze line location



March 24, 1970 J. w. CARSON 3,502,133

CONTINUOUS CASTING METHOD AND APPARATUS FOR CONTROLLING FREEZE LINELOCATION Filed March 5, 1967 ms ATTORNEYS United States Patent Int. Cl.B22c US. Cl. 164-4 16 Claims ABSTRACT OF THE DISCLOSURE A continuouscasting method and apparatus involving sensing temperature variations inthe mold adjacent the surface of the incipient ingot being continuouslycast in the region of the freeze line, especially in the casting of aningot, such as of aluminum, at least about 6 inches thick and having awidth to thickness ratio of at least about 3 to l, and responsively tothe sensed temperature variations controlling the application, as byspraying, of coolant, normally water, to minimize inward and outwardmovement of the freeze line. The sensing means, which may include athermocouple, are preferably located opposite a wide face of theincipient ingot and at a location closer to the corner than to thecenter of the wide face and close enough to the corner to sense thetemperature variations in the region of the corner. The coolant ispreferably applied to the mold or/and ingot only at the wide faces ofthe ingot.

This invention relates to a continuous casting method and apparatus. Itrelates particularly to continuous casting involving regulation of thecooling of the metal being cast to minimize inward and outward movementof the freeze line and by so doing promote uniformity and superiorquality of the casting produced.

Continuous casting of metal, both ferrous and nonferrous, is known inthe art but difiiculty has been experienced in obtaining optimum qualityand uniformity of product. A coolant, normally water, is employed tocool the metal as it solidifies. The relatively narrow zone between theliquidus and solidus isotherms, in which the metal changes state fromliquid or molten to solid is known as the liquid-solid interface orfreeze-line. If the freeze line can be maintained at a substantiallyconstant location optimum results are obtained. Prior efforts to somaintain the freeze line have not been entirely successful.

Variations in the temperature of the feed metal, especially due tofluctuations of furnace temperature within the limits of practicalcontrol, are sometimes so great as to move the freeze line inwardly (inthe direction opposite the direction of advance of the metal duringcasting) with resultant sticking and tearing of the ingot, or outwardly(in the direction of advance of the metal during casting) with resultantcolds shutting and lapping.

Other factors can cause or contribute to inward and outward movement ofthe freeze line, such as variations in the casting rate or in thetemperature of the coolant. Operators try to offset these variables whenthey can be detected, as by adjusting the rate of application ofcoolant, but such adjustments involve the human factor and cannot berelied upon to stabilize the location of the freeze line.

Particularly troublesome is the start-up portion of the castingoperation in which the outer or lower end of the mold (depending onwhether it is arranged vertically or horizontally) is closed off by aso-called starter block and the mold is filled with molten metal afterwhich intensive cooling of the mold is required before casting ice of aningot can be initiated by withdrawing the starter block. The rate ofapplication of coolant during this phase of the casting operation shouldbe much greater than is necessary during subsequent steady-stateoperation. The problem is to know how much to increase the cooling rateand for how long before reducing the cooling rate to the point that thefreeze line does not move upwardly or inwardly too far while still beingreasonably assured that the frozen shell of metal adjacent the mold wallat the outlet end of the mold will not rupture and release the moltenmetal. In the past this has been accomplished only by reliance on theoperators skill.

The effect of each of the factors above mentioned may vary from onecasting operation to another, particularly where different ingot sizesand different alloys are involved.

I have devised an improved system of control whereby a more uniform andsuperior quality ingot can be produced than has heretofore been possibleon a regular basis. I have discovered that detectable changes in thetemperature of a portion of the mold close to the surface of theincipient ingot in the region of the freeze line can be used to effectcontrol of inward and outward movement of the freeze line. I provide forlocating a temperature sensing element in the mold wall and employingvariations in the sensed temperature to control the casting operation.

My invention is especially well adapted for improving continuous castingoperation in the production of large rectangular ingots such, forexample, as ingots of aluminum or aluminum alloy having a transversesection several feet in width and at least about six inches inthickness, especially such ingots having a width to thickness ratio onthe order of 3 to 1 or more. The casting of generally rectangular ingotshaving a width and thickness suitable for continuous rolling without thenecessity of employing the usual ingot slabbing or breakdown operationsoffers considerable advantage over conventional procedures. A majordifficulty in the casting of such ingots has been the grossly unevencooling rate which results when the coolant is applied peripherally ofthe mold in a uniform manner. This is due to the relatively greatcooling effect at the corners of the ingot and the consequent difficultyof balancing heat extraction around the periphery of the ingot. Thus thecooling rate for an increment of metal along the longitudinal axis ofthe mold (at the center of the ingot) is considerably less than that forincrements of metal near the surface of the ingot.

I have discovered that in such cases coolant should be applied to thewide faces of the ingot or/ and the mold walls at the wide faces of theingot and not at all (or at least at a reduced rate) to the relativelynarrow end faces of the ingot or/ and the mold walls at the relativelynarrow end faces of the ingot. I control the solidification rate of themetal by varying the rate of applying coolant to the wide faces of theingot or the adjacent walls of the mold or both.

I provide a continuous casting method comprising feeding molten metalinto the entrance end of a mold, effecting solidification of the metalduring its passage through the mold, withdrawing solidified metal fromthe exit end of the mold, applying coolant to at least one of the moldand metal to promote solidification of the metal, the juncture of moltenand solidified portions of the metal being cast defining a so-calledfreeze line ther'ebetween, sensing temperature variations in the regionof the freeze line, an increase in temperature being associated withoutward movement of the freeze line away from the entrance end of themold and a decrease in temperature being associated with inward movementof the freeze line away from the exit end of the mold, and responsivelyto the sensed temperature variations controlling the application of thecoolant to minimize inward and outward movement of the freeze line. Thecoolant is normally water; it may be applied to the mold in the regionof the freeze line or directly against the solidified metal emergingfrom the mold exit or both.

I further provide a method of continuously casting an ingot at leastabout six inches thick and having a width to thickness ratio of at leastabout 3 to 1 comprising feeding molten metal into the entrance end of amold, efiecting solidification of the metal during its passage throughthe mold, withdrawing solidified metal from the exit end of the mold,applying coolant to at least one of the mold and metal at least at thewide faces of the ingot to promote solidification of the metal, thejuncture of molten and solidified portions of the metal being castdefining a so-called freeze line therebetween, sensing temperaturevariations in the region of the freeze line, an increase in temperaturebeing associated with outward movement of the freeze line away from theentrance end of the mold and a decrease in temperature being associatedwith inward movement of the freeze line away from the exit end of themold, and responsively to the sensed temperature variations controllingapplication of the coolant to minimize inward and outward movement ofthe freeze line. The coolant may, as above indicated, be applied only tothe wide faces of the ingot or/and the mold walls at the wide faces ofthe ingot. Desirably the temperature variations are sensed at a wideface of the ingot at a location closer to a corner than to the center ofthe wide face and close enough to the corner to sense the temperaturevariations in the region of the corner.

I still further provide continuous casting apparatus comprising a moldhaving a passageway therethrough, means for feeding molten metal intothe entrance end of said passageway, means for withdrawing solidifiedmetal from the exit end of said passageway, means for applying coolantto at least one of the mold and metal to promote solidification of themetal, the juncture of molten and solidified portions of the metal beingcast defining a socalled freeze line therebetween, means sensingtemperature variations in the region of the freeze line, an increase intemperature being associated with outward movement of the freeze lineaway from the entrance end of said passageway and a decrease intemperature being associated with inward movement of the freeze lineaway from the exit end of said passageway, and means responsive to thesensing means controlling the application of the coolant to minimizeinward and outward movement of the freeze line. The coolant applyingmeans are desirably constructed and arranged to apply the coolant to themold in the region of the freeze line or directly against the solidifiedmetal emerging from the exit end of the passageway or both.

I also provide continuous casting apparatus comprising a mold having apassageway therethrough shaped to cast an ingot at least about sixinches thick and having a width to thickness ratio of at least about 3to 1, means for feeding molten metal into the entrance end of saidpassageway, means for withdrawing solidified metal from the exit end ofsaid passageway, means for applying coolant to at least one of the moldand metal at least at the wide faces of the ingot to promotesolidification of the metal, the juncture of molten and solidifiedportions of the metal being cast defining a so-called freeze linetherebetween, means sensing temperature variations in the region of thefreeze line, an increase in temperature being associated with outwardmovement of the freeze line away from the entrance end of saidpassageway and a decrease in temperature being associated with inwardmovement of the freeze line away from the exit end of said passageway,and means responsive to the sensing means controlling the application ofthe coolant to minimize inward and outward movement of the freeze line.The coolant applying means may be constructed and arranged to apply thecoolant only to the wide faces of the ingot or/ and the mold walls atthe wide faces of the ingot. The sensing means are desirably locatedopposite a wide face of the ingot at a location closer to a corner thanto the center of the wide face and close enough to the corner to sensethe temperature variations in the region of the corner.

Other details, objects and advantages of the invention will becomeapparent as the following description of a present preferred embodimentthereof and a present preferred method of practicing the same proceeds.

In the accompanying drawings I have shown a present preferred embodimentof the invention and have illustrated a present preferred method ofpracticing the same in which FIGURE 1 is a diagrammatic plan view of acontinuous casting mold to which my invention is applied; and

FIGURE 2 is an enlarged vertical cross-sectional view taken on the lineIIII of FIGURE 1.

Referring now more particularly to the drawings, there is shown a mold 2for continuous casting, the mold being of generally rectangular shapewith rounded corners and being of such size that the cast ingot is about6 inches thick (the horizontal dimension in FIGURE 1) while its width(the vertical dimension in FIGURE 1) is of the order of at least threetimes the thickness. The mold is lined with a wall 3 of insulatingmaterial (as, for example, as shown in United States Patent No.2,983,972) just above the freeze line. The freeze line is indicated at 4in FIGURE 2.

A coolant spray system is provided including pipes 5 disposedhorizontally alongside the mold with orifices 6 for spraying coolant,normally water, onto the outside of the mold and also onto the castingemerging from the mold as shown in FIGURE 2. The coolant is sprayed onlyonto the long sides of the mold and the wide faces of the ingot and noton the short sides of the mold or the narrow faces of the ingot. Thepipes 5 are connected by pipes 7 which contain no orifices leading to aT 8 from which a pipe 9 leads to a valve 10 and thence to a coolantsupply pipe 11. A by-pass 12 extends around the valve 10 with a valve 13in the by-pass.

I provide a temperature sensing element 14 in the wall of the mold justbelow the bottom of the wall 3 of insulating material and reachingalmost but not quite to the inner surface of the mold wall 2 as shown inFIGURE 2. The sensing element 14 is, as shown in FIGURE 1, locatedopposite a wide face of the ingot at a location closer to a corner thanto the center of the wide face and close enough to the corner to sensethe temperature variations in the region of the corner. The sensingelement 14 may be a thermocouple transmitting a signal to aproportioning control 15 controlling a motor operator 16 for the by-passvalve 13. The valve 10 is manually set and the valve 13 is automaticallycontrolled by the thermocouple 14. Increase in temperature sensed by thethermocouple results in opening somewhat the valve 13 with consequentincreased flow of coolant through the system and onto the wide faces ofthe mold and ingot. Decrease in temperature sensed by the thermocoupleresults in closing somewhat the valve 13 with consequent decreased Howof coolant through the system and onto the wide faces of the mold andingot.

As mentioned above and as shown in FIGURE 2, the coolant is sprayed ontoboth the mold and the ingot at the wide faces of the ingot. It impingesupon the ingot emerging from the mold at 17 (see FIGURE 2). Impingementof the coolant at point 17 establishes an ingot surface temperature thatcreates the driving force for heat flow, particularly the latent heat offusion of the solidifying metal. This driving force tends to act in anare near the ingot surface, especially when there is minor heat flow tothe upper region of the mold. If true equilibrium exits the temperatureof the mold in the region where the freeze line reaches the wide face ofthe ingot should be constant.

When the temperature at 17 decreases an increased driving force for heatflow is created from the large heat of fusion reservoir at the yetunchanged liquid-solid intcrface or freeze line. Heat flow isaccelerated along the shorter parts and liquid solidifies at a higherpoint, possibly within the wall 3. The mold temperature opposite thefreeze line tends to approach the coolant temperature.

When the temerature at 17 increases the driving force for heat flow fromthe large heat of fusion reservoir is reduced. Since more of the mold isin contact with the plastic ingot mass the mold temperature opposite thefreeze line increases. The mold temperature measurement at the locationof the thermocouple 13 is an indirect method of gauging the thermalequilibrium of the ingot, particularly in the vicinity of the ingotsurface. A refinement in heat transfer balancing is achieved byutilizing that measurement in conjunction with the automatic flowcontrol system for coolant application as above described.

While I have shown and described a present preferred embodiment of theinvention and have illustrated a present preferred method of practicingthe same, it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied and practicedwithin the scope of the following claims.

I claim:

1. A continuous casting method comprising feeding molten metal into theentrance end of a mold, effecting solidification of the metal during itspassage through the mold, withdrawing solidified metal from the exit endof the mold, applying coolant to at least one of the mold and metal topromote solidification of the metal, the juncture of molten andsolidified portions of the metal being cast defining a so-called freezeline therebetween, sensing temperature variations of the mold in theregion of the freeze line, an increase in temperature being associatedwith outward movement of the freeze line away from the entrance end ofthe mold and a decrease in temperature being associated with inwardmovement of the freeze line away from the exit end of the mold, andresponsively to the sensed temperature variations controlling theapplication of the coolant to minimize inward and outward movement ofthe freeze line.

2. A continuous casting method as claimed in claim 1 in which thecoolant is applied to the mold in the region of the freeze line.

3. A continuous casting method as claimed in claim 1 in which thecoolant is applied directly against the solidified metal emerging fromthe mold exit.

4. A continuous casting method as claimed in claim 1 in which thecoolant is applied both to the mold in the region of the freeze line anddirectly against the solidified metal emerging from the mold exit.

5. A method of continuously casting an ingot at least about six inchesthick and having a width to thickness ratio of at least about 3 to 1comprising feeding molten metal into the entrance end of a mold,effecting solidification of the metal during its passage through themold, withdrawing solidified metal from the exit end of the mold,applying coolant to at least one of the mold and metal at least at the*wide faces of the ingot to promote solidification of the metal, thejuncture of molten and solidified portions of the metal being castdefining a so-called freeze line therebetween, sensing temperaturevariations of the mold in the region of the freeze line, an increase intemperature being associated with outward movement of the freeze lineaway from the entrance end of the mold and a decrease in temperaturebeing associated with inward movement of the freeze line away from theexit end of the mold, and responsively to the sensed temperaturevariations controlling application of the coolant to minimize inward andoutward movement of the freeze line.

6. A method of continuously casting an ingot as claimed in claim 5 inwhich coolant is applied only at the wide faces of the ingot.

7. A method of continuously casting an ingot as claimed in claim 5 inwhich the temperature variations are sensed at a wide face of the ingotat a location closer to a corner than to the center of the wide face andclose enough to the corner to sense the temperature variations in theregion of the corner.

8. A method of continuously casting an ingot as claimed in claim 6 inwhich the temperature variations are sensed at a wide face of the ingotat a location closer to a corner than to the center of the wide face andclose enough to the corner to sense the temperature variations in theregion of the corner.

9. Continuous casting apparatus comprising a mold having a passagewaytherethrough, said mold being adapted to receive molten metal fed intothe entrance end of said passageway, the solidified metal beingwithdrawn from the exit end of said passageway, means for applyingcoolant to at least one of the mold and metal to promote solidificationof the metal, the juncture of molten and solidified portions of themetal being cast defining a socalled freeze line therebetween, means forsensing temperature variations of the mold in the region of the freezeline, the mold having a heat insulating interior lining spaced inwardlyfrom the exit end of said passageway and extending along the passagewaytoward the entrance end thereof, said sensing means being located in themold wall adjacent the outer end of said interior lining at a zone wherethe metal being cast is in contact with the mold wall, and control meansresponsive to said temperature sensing means for counteracting suchtemperature variations of the mold.

10. Continuous casting apparatus as claimed in claim 9 in which thecoolant applying means are constructed and arranged to apply the coolantto the mold in the region of the freeze line.

11. Continuous casting apparatus as claimed in claim 9 in which thecoolant applying means are constructed and arranged to apply the coolantdirectly against the solidified metal emerging from the exit end of thepassageway.

12. Continuous casting apparatus as claimed in claim 9 in 'which thecoolant applying means are constructed and arranged to apply the coolantboth to the mold in the region of the freeze line and directly againstthe solidified metal emerging from the exit end of the passageway.

13. Continuous casting apparatus comprising a mold having a passagewaytherethrough shaped to cast an ingot at least about six inches thick andhaving a width to thickness ratio of at least about 3 to 1, said moldbeing adapted to receive molten metal fed into the entrance end of saidpassageway, the solidified metal being withdrawn from the exit end ofsaid passageway, means for applying coolant to at least one of the moldand metal at least at the wide faces of the ingot to promotesolidification of the metal, the juncture of molten and solidifiedportions of the metal being cast defining a so-called freeze linetherebetween, means for sensing temperature variations of the mold inthe region of the freeze line, the mold having a heat insulatinginterior lining spaced inwardly from the exit end of said passageway andextending along the passageway toward the entrance end thereof, saidsensing means being located in the mold wall adjacent the outer end ofsaid interior lining at a zone where the metal being cast is in contactwith the mold wall, and control means responsive to said temperaturesensing means for counteracting such temperature variations of the mold.

14. Continuous casting apparatus as claimed in claim 13 in which thecoolant applying means are constructed and arranged to apply the coolantonly at the wide faces of the ingot.

15. Continuous casting apparatus as claimed in claim 13 in which thesensing means are located opposite a wide face of the ingot at alocation closer to a corner than to the center of the Wide face andclose enough to the corner to sense the temperature variations in theregion of the corner.

16. Continuous casting apparatus as claimed in claim 14 in which thesensing means are. located opposite a wide face of the ingot at alocation closer to a corner than to the center of the Wide face andclose enough to the corner to sense the temperature variations in theregion of the corner. 1

References Cited UNITED STATES PATENTS 3,204,460 9/1965 Milnes 164277X3,283,370 11/1966 Jendraszkiewicz et a1. 164'154 15 8 Tiskus et a1164l55 Eldred 164-89 Webster 164155 Thalmann 164-89 X Adams 164-155FOREIGN PATENTS Austria.

WILLIAM J1 STEPHENSON, Primary Examiner R. SPENCER ANNEAR, AssistantExaminer US. Cl. X.R.

